I took the suggest of another earlier post on a similar topic, and have ordered small-mesh filter socks for my sump overflow line, and I'm waiting for that to arrive, but I've still got an amazing amount of crud in my sump. Seems like everything settles there. Looks like a light-brown power at the bottom. Particularly in the first chamber where the skimmer is.

I have the same exact problem with my sump setup....whenver i do a PWC I try to clean as much out as I can...but my sump is also wide open and only two chamber. The snails sound like a good idea because most of this is detritus build-up....I have the same filter sock on mine, but it only makes it a finer powder. Good luck to you!